Sulfur represents an impurity that is frequently encountered in many commercial catalytic processes, such as hydrogenation, hydrocracking, reforming, methanation, and synthesis. Unfortunately, these sulfur impurities (H2S, RSH, RSSR, etc.) are known to act as a poison for catalytic processes that utilize a reduced metal as the primary active phase. A poison will typically impair the performance of a heterogeneous catalyst system by reducing the systems overall catalytic activity. The presence of sulfur impurities may cause a significant decrease in catalytic activity even at very low concentrations due to their ability to adsorb onto sites at the surface of the metal catalyst and support material and due to their ability to form very stable adsorbed species under a variety of different reaction conditions. The formation of stable adsorbed species prevents the reactant molecules from accessing catalytically active sites, thereby, decreasing the overall catalytic activity for the catalyst system.
One specific example, of a catalytic process that is susceptible to sulfur poisoning is the catalytic treatment of exhaust gases arising from a combustion engine, such as a diesel engine. Diesel fuel typically contains a significant amount of sulfur, which when burnt, is converted to sulfur compounds such as sulfur dioxide. Sulfur dioxide in the exhaust gas oxidizes the catalyst support material yielding sulfur trioxide which reacts with water to create a sulfate such as sulfuric acid. Sulfates are not able to revert back to a gas. This creates a buildup of sulfates on the catalyst support material and leads to the loss of acidic active sites. The loss of acidic active sites on the catalyst support material reduces the activity of the overall catalyst system. In addition sulfur contamination of the catalyst support material may also result in decreased surface area due to blocking of pores in the catalyst structure.